This invention relates to optical waveguide amplifiers, typically, but not exclusively, erbium doped optical waveguide amplifiers, and particularly to gain-shaped optical waveguide amplifiers. Gain-shaping in an optical waveguide amplifier is employed to provide a more nearly flattened spectral gain characteristic than would otherwise occur, and may be accomplished by the use of a notch filter as for instance described by M. Tachibana et al in a paper entitled `Erbium-Doped Fiber Amplifier with Flattened Gain Spectrum` IEEE Photonics Technology Letters, Volume No. 2, February 1991 pages 118-120, to which attention is directed. In the absence of smoothing, the gain characteristic of an optical waveguide amplifier is liable to exhibit a relatively sharply defined peak to the long wavelength side of which is a broader plateau of reduced gain. In the case of a typical erbium doped optical fibre amplifier pumped at 980 nm, this peak is at about 1535 nm. A disadvantage of the notch filter described in the above referenced paper is that, because it relies upon the use of a mechanical grating to induce mode coupling between a propagating mode and a radiating mode whose propagation constant differs by only a small amount from that of the propagating mode, it needs to be inconveniently long in order to achieve the requisite selectivity. In the particular example quoted in that paper the mechanical grating extends linearly 390 mm.